Photoacoustic apparatus, signal processing method of photoacoustic apparatus, and program

ABSTRACT

A photoacoustic apparatus includes a light source generating multiple lights with different wavelengths, a converting element receiving photoacoustic waves generated in a subject by the subject being irradiated by the multiple lights, a first distribution obtaining unit obtaining a property distribution based on optical absorption within the subject, for each of the mutually different wavelengths, using time-sequence reception signals each output from the converting element for each of the mutually different wavelengths, a second distribution obtaining unit obtaining, using multiple property distributions based on optical absorption for each wavelength, a concentration-related distribution of a substance of an object region of the subject, and a statistical information obtaining unit obtaining statistical information indicating variance in distribution in at least part of the concentration-related distribution.

BACKGROUND Field

Aspects of the present invention generally relate to a photoacousticapparatus for obtaining information of inside a subject, a signalprocessing method, and a program, and more particularly relates totechnology using photoacoustic waves generated by a subject beingirradiated by light.

Description of the Related Art

Research regarding imaging of functional information, which isphysiological information of a living body, has come to be performed inthe medical field in recent years. One technology in imaging offunctional information is photoacoustic imaging (PAI).

In photoacoustic imaging, a subject is first irradiated by pulse lightgenerated from a light source. The irradiation light propagates withinthe subject and diffuses. At multiple locations within the subject,energy of this light is absorbed, generating acoustic waves (hereinafterreferred to as “photoacoustic waves”). The photoacoustic waves arereceived by converting elements, and the reception signals are analyzedand processed by a processor, thereby yielding distribution relating tooptical property values within the subject as image data.

Distribution relating to optical property values are yielded in the formof sound pressure generated by optical absorption (initial soundpressure distribution), optical absorption coefficient distribution, andso forth. Also, irradiation using multiple pulse lights of differentwavelengths and obtaining the optical absorption coefficients for eachwavelength enables a concentration-related distribution of substanceswithin the subject (a distribution of values relating to concentrationof substances) to be obtained as a distribution relating to opticalproperty values.

As for a concentration related distribution, there is the distributionof rate of content of oxyhemoglobin as to the total hemoglobin in theblood, i.e., blood oxygen saturation distribution. The opticalabsorption spectrums of deoxyhemoglobin and oxyhemoglobin are not thesame. Accordingly, calculation of oxygen saturation uses the principlethat the rate of content of each can be found by comparing the spectrumsmeasured at different wavelengths, as disclosed in Hao F. Zhang, et al.“Functional photoacoustic microscopy for high-resolution and noninvasivein vivo imaging” Nature Biotechnology 24, 848-851 (July 2006).

At the time of performing tests by photoacoustic imaging, body movementoccurs if the subject is a living body, due to breathing, pulse,actions, and so forth. When body movement occurs, positional deviationoccurs between the initial sound pressure distributions calculated atdifferent wavelengths, or between absorption coefficient distributionscalculated at different wavelengths. Positional deviation amongwavelengths can occur due to movement of a probe, in addition to bodymovement of the subject. The relatively positional deviation between thesubject and the probe is a factor in lowering calculation accuracy ofthe oxygen saturation distribution.

PCT Japanese Translation Patent Publication No. 2010-512929 disclosesdetecting movement of tissue based on ultrasound images obtained bytransmitting ultrasound waves and receiving the reflected waves, therebyestimating deformation of objects among the photoacoustic images.

The movement detection method disclosed in PCT Japanese TranslationPatent Publication No. 2010-512929 is performed based on ultrasoundimages. Ultrasound images obtained by ultrasound echoes are distributionimages reflecting difference in acoustic impedance, with structureswhere acoustic wave reflectance is great being visualized. On the otherhand, structures where the rate of absorption is high are visualized indistributions regarding optical property values indicated by thephotoacoustic image. Thus, the object of imaging is not the same inultrasound images and photoacoustic images, so there is a possibilitythat reflecting movement detected based on ultrasound images inpositional deviation among photoacoustic images may not be able tocorrect position deviation with high accuracy.

SUMMARY

A photoacoustic apparatus includes a light source configured to generatea plurality of lights with mutually different wavelengths, a convertingelement configured to receive photoacoustic waves generated in a subjectby the subject being irradiated by the plurality of lights, a firstdistribution obtaining unit configured to obtain a property distributionbased on optical absorption within the subject, for each of the mutuallydifferent wavelengths, using time-sequence reception signals output fromthe converting element for each of the mutually different wavelengths; asecond distribution obtaining unit configured to, using a plurality ofthe property distributions based on optical absorption for each of themutually different wavelengths, obtain a concentration-relateddistribution of a substance of an object region of the subject, and astatistical information obtaining unit configured to obtain statisticalinformation indicating variance in distribution in at least a part ofthe concentration-related distribution. The second distributionobtaining unit obtains, based on the statistical information, aconcentration-related distribution where positional deviation betweenthe object region and the converting element at each of the mutuallydifferent wavelengths is suppressed.

Further features of the present disclosure will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating the configuration of aphotoacoustic apparatus according to an exemplary embodiment.

FIG. 2 is a flowchart illustrating operations of a signal processingunit according to an exemplary embodiment.

FIG. 3 is a schematic diagram illustrating an example of a display imageaccording to an exemplary embodiment.

FIG. 4 is a schematic diagram for describing a way to obtain dispersion.

FIG. 5 is a schematic diagram for describing a configuration example ofa signal processing unit.

FIG. 6 is a schematic diagram for describing a method for decidingcoordinate shift and positional deviation.

FIG. 7 is a schematic diagram illustrating deviation between a peakwithin a 756 nm absorption coefficient distribution and a peak within a797 nm absorption coefficient distribution.

FIG. 8 is a diagram illustrating dispersion within an oxygen saturationdistribution in a case where there is positional deviation.

FIG. 9 is a diagram illustrating dispersion within an oxygen saturationdistribution in a case where there is no positional deviation.

FIG. 10 is a schematic diagram illustrating the configuration of aphotoacoustic apparatus according to a fourth embodiment.

FIG. 11 is a flowchart illustrating operations of the signal processingunit in the fourth embodiment.

FIG. 12 is a schematic diagram illustrating the configuration of aphotoacoustic apparatus according to a fifth embodiment.

FIG. 13 is a flowchart illustrating operations of the signal processingunit in the fifth embodiment.

FIG. 14 is a diagram illustrating the way in which imaging is performedin the fifth embodiment.

DESCRIPTION OF THE EMBODIMENTS

Exemplary embodiments will be described with reference to the drawings.Components that are the same will generally be denoted by the samereference numerals and description thereof omitted.

The photoacoustic apparatus according to an aspect of the presentinvention obtains information within a subject (information relating tooptical property values) by using reception signals obtained byreceiving photoacoustic waves. The term “photoacoustic waves” in thepresent Specification means acoustic waves generated by absorbing light,and includes so-called “acoustic waves”, “ultrasound waves”, “soundwaves”, “elastic waves”, and “photo-ultrasound waves”, generated byabsorbing light.

Information relating to optical property values obtained in an aspect ofthe present invention reflects the rate of absorption of optical energy.Specific optical property values include “sound pressure (typicallyinitial sound pressure)” of generated acoustic waves, “optical energyabsorption density” derived from sound pressure, “absorptioncoefficients”, “information relating to concentration(concentration-related information)” of a substance making up tissue,and so forth.

“Concentration-related information” includes values related toconcentration of a substance present in the subject, that is obtainedusing “property distribution based on optical absorption” of multiplewavelengths. Specifically, “concentration-related information” is“oxygen saturation”, “a value where oxygen saturation has been weightedby intensity of adsorption coefficients or the like”, “total hemoglobinconcentration”, “oxyhemoglobin concentration”, “deoxyhemoglobinconcentration”, and so forth. “Concentration-related information” mayalso be “glucose concentration”, “collagen concentration”, “melaninconcentration”, “volume fraction” of fat and water, and so forth.

The apparatus according to an aspect of the present invention cangenerate two-dimensional or three-dimensional distribution data, byobtaining optical property values at multiple positions. That is to say,“sound pressure distribution”, “optical energy absorption densitydistribution”, “absorption coefficient distribution”,“concentration-related distribution”, and so forth can be obtained. Theobtained distribution data can be generated as image data.

Note that “sound pressure distribution”, “optical energy absorptiondensity distribution”, and “absorption coefficient distribution” will becollectively referred to “property distribution based on opticalabsorption” in the present Specification. The “concentration-relateddistribution is obtained based on the “property distribution based onoptical absorption” of multiple wavelengths.

The photoacoustic apparatus according to the following embodimentsenables diagnosis of malignancy and blood vessel diseases in humans andanimals, and for follow-up in chemotherapy. Accordingly, part of theliving body that is the subject, more specifically one region of thehuman or animal (breast, body organs, circulatory organs, digestivesystem, bones, muscle, fat, etc.), is presumed to be the inspectiontarget. Substances which are inspection targets include hemoglobin,glucose, water within the body, melanin, collagen, fat, and so forth.Further, radiopaque dye such as indocyanine green (ICG) administrated tothe body may be the inspection target. It is sufficient that theinspection target has a characteristic optical absorption spectrum.

Exemplary embodiments will now be described. Note that the exemplaryembodiments are not restricted to the photoacoustic apparatus itself,but also encompass a controlling method thereof, and a program forexecution thereof.

First Embodiment

The following is a description of the configuration and processingthereof of a photoacoustic apparatus according to a first embodiment.

Overall Apparatus Configuration

FIG. 1 is a schematic diagram illustrating the configuration of thephotoacoustic apparatus according to the present embodiment. Thephotoacoustic apparatus according to the present embodiment includes atleast a light source 1, a probe 30 that has a converting element 3 whichreceives photoacoustic waves, and a signal processing unit 40 thatperforms signal processing using reception signals output from theconverting element 3.

A subject 2 is irradiated by light output from the light source 1 andpassing through an optical propagation member (omitted fromillustration) such as fiber, lenses, and so forth. Note that the subjectis irradiated by multiple pulse lights having different wavelengths fromeach other, at separate timings. The irradiation light propagates withinthe subject and diffuses, and is absorbed by substances present withinthe subject. Such substances which absorb light each absorb opticalenergy of each wavelength, and generate photoacoustic waves. That is tosay, light of a first wavelength generates first photoacoustic waves,and light of a second wavelength generates second photoacoustic waves.The generated photoacoustic waves propagate through the subject andreach the converting element 3. The converting element 3 is provided soas to acoustically match the subject.

Each of multiple converting elements 3 output reception signals intime-sequence, by receiving the photoacoustic waves. That is to say, theconverting elements 3 output first reception signals in time-sequence byreceiving first photoacoustic waves, and output second reception signalsin time-sequence by receiving second photoacoustic waves. The outputreception signals are input to the signal processing unit 40. Receptionsignals are sequentially input to the signal processing unit 40, foreach irradiated pulse light. The signal processing unit 40 generatesdistributions such as property distribution and concentration-relateddistribution, based on optical adsorption within the subject, using theinput reception signals. The signal processing unit 40 also generatesimage data based on the generated distributions, and images can bedisplayed on a display unit 8. Input of region settings and so forthaccepted from a user (operator such as a physician or technician) via aninput unit 12.

In a case where the photoacoustic apparatus takes relatively smallsubjects as inspection targets, such as in a case of a photoacousticmicroscope or the like, the number of converting elements 3 that theprobe 30 has may be one. However, in a case where the photoacousticapparatus takes a relatively large subject, such as a breast or thelike, as the inspection target, multiple converting elements 3 arepreferably provided to the probe 30.

Internal Configuration of Signal Processing Unit 40

Next, the configuration within the signal processing unit 40 accordingto the present embodiment will be described. The signal processing unit40 according to the present embodiment has a signal collecting unit 9, afirst distribution obtaining unit 4, a second distribution obtainingunit 5, a display control unit 10, and a control unit 11. The controlunit 11 includes a region setting unit 13, a statistical informationobtaining unit 6, a shift unit 7, and a deciding unit 14. The processingflow within the signal processing unit 40 will be described later withreference to FIG. 2, and the components within the signal processingunit 40 will be described first.

The signal collecting unit 9 collects time-sequence analog receptionsignals output from each of the multiple converting elements 3, by eachchannel, and performs signal processing such as amplifying the receptionsignals, AD conversion of the received analog signals, storage ofdigitized reception signals, and so forth.

The first distribution obtaining unit 4 generates property distributionsbased on optical absorption within the subject, using the receptionsignals output from the signal collecting unit 9. The followingdescription relates to an example of obtaining absorption coefficientdistribution as a property distribution based on optical absorption. Anabsorption coefficient μ_(a) at a position (i, j, k) within the subjectcan be obtained by Expression (1). Note that i, j, and k are eachintegers representing coordinates in the subject.P=Γ·μ _(a)·ϕ  (1)where P represents initial sound pressure (generated sound pressure), Γrepresents the Grueneisen constant, and ϕ represents the quantity oflight which has reached the position (i, j, k).

Note that the initial sound pressure at the position (i, j, k) on thethree-dimensional spatial coordinates is obtained by imagereconstruction, by filtering based on the reception signals for eachchannel that are output from the signal collecting unit 9, using a bandcorrection filter of the probe. Examples of image reconstruction whichmay be used include known reconstruction techniques, such as universalback projection (UBP) described in U.S. Pat. No. 5,713,356, filteredback projection (FBP), and so forth. Delay-and-sum processing may alsobe used.

Performing this image reconstruction processing at each position obtainsthe initial sound pressure at each position, so the initial soundpressure distribution can be obtained. The initial sound pressuredistribution may be three-dimensional distribution data (set data ofvoxels) corresponding to a certain region within the subject, or may betwo-dimensional distribution data (set data of pixels) corresponding toone cross-section thereof.

Note that optical focusing photoacoustic microscopes, and acousticfocusing photoacoustic microscopes using focusing probes, can generatedistribution data without performing image reconstruction processing.Specifically, the probe 30 and a light irradiation spot are movedrelative to the subject 2 by a scanning mechanism (omitted fromillustration), and the probe 30 receives photoacoustic waves at multiplescanning positions. After having performed envelope detection regardingtime change of the obtained reception signals, the first distributionobtaining unit 4 performs conversion of the temporal axis direction ofthe signals in each optical pulse into the depth direction, and plotsthis on spatial coordinates. Distribution data can be configured byperforming this at every scan position.

The first distribution obtaining unit 4 obtains the absorptioncoefficient distribution using Expression (1), based on the initialsound pressure obtained in this way. Note that the Grueneisen constantcan be considered to be constant. The light quantity φ may be consideredto be constant in the subject, but the light quantity distribution ispreferably obtained by performing calculation taking into considerationoptical propagation within the subject, from the light quantitydistribution input to the subject, in order to obtainconcentration-related information more accurately. Accordingly, thefirst distribution obtaining unit 4 obtains the absorption coefficientdistribution using the initial sound pressure and the light quantitydistribution. Thus, the first distribution obtaining unit 4 according tothe present embodiment obtains the absorption coefficient distributionfor each of the multiple wavelengths emitted from the light source 1,and outputs to the second distribution obtaining unit 5.

The second distribution obtaining unit 5 uses multiple absorptioncoefficient distributions for each wavelength output from the firstdistribution obtaining unit 4, and generates concentration-relateddistribution. An example of obtaining oxygen saturation distribution asthe concentration-related distribution will be described below.

Assuming that optical absorption other than due to hemoglobin isnegligible at wavelength λ₁ and wavelength λ₂, the absorptioncoefficients for wavelength λ₁ and wavelength λ₂ are each expressed asin Expressions (2) and (3), using the molar absorption coefficient foroxyhemoglobin and the molar absorption coefficient for deoxyhemoglobin.μ_(a)(λ₁)=ε_(ox)(λ₁)C _(ox)+ε_(de)(λ₁)C _(de)  (2)μ_(a)(λ₂)=ε_(ox)(λ₂)C _(ox)+ε_(de)(λ₂)C _(de)  (3)

In these Expressions, μ_(a) (λ₁) represents the absorption coefficientof light of the wavelength λ₁ at position (i, j, k), μ_(a)(λ₂)represents the absorption coefficient of light of the wavelength λ₂ atposition (i, j, k), in units of mm⁻¹. C_(ox) is the amount ofoxyhemoglobin in mols, and C_(de) is the amount of deoxyhemoglobin inmols. Each represent values at position (i, j, k).

ε_(ox)(λ₁) and ε_(de)(λ₁) represent the molar absorption coefficient(mm⁻¹mol⁻¹) of oxyhemoglobin and deoxyhemoglobin at wavelength λ₁,respectively, and ε_(ox)(λ₂) and ε_(de)(λ₂) represent the molarabsorption coefficient (mm⁻¹mol⁻¹) of oxyhemoglobin and deoxyhemoglobinat wavelength λ₂, respectively. ε_(ox)(λ₁), ε_(de)(λ₁), ε_(ox)(λ₂), andε_(de)(λ₂) can be found beforehand by measurement or literature values.

Accordingly, C_(ox) and C_(de) are each obtained using the molarabsorption coefficient, μ_(a) (λ₁), and μ_(a)(λ₂), by solving thesimultaneous equations in Expressions (2) and (3). In a case where thenumber of wavelengths used is great, the least square method ispreferably used. The oxygen saturation SO₂ is defined as the rate ofoxyhemoglobin in total hemoglobin, as shown in Expression (4).Accordingly, the oxygen saturation SO₂ can be shown by Expression (5),based on Expressions (2), (3), and (4).

Thus, the second distribution obtaining unit 5 can obtain the oxygensaturation SO₂ at position (i, j, k) based on the molar absorptioncoefficient, μ_(a)(λ₁), and μ_(a)(λ₂), using the Expression (5).

$\begin{matrix}{{SO}_{2} = \frac{C_{ox}}{C_{ox} + C_{de}}} & (4) \\{{SO}_{2} = \frac{{\frac{\mu_{a}\left( \lambda_{2} \right)}{\mu_{a}\left( \lambda_{1} \right)} \cdot {ɛ_{de}\left( \lambda_{1} \right)}} - {ɛ_{de}\left( \lambda_{2} \right)}}{\left( {{ɛ_{ox}\left( \lambda_{2} \right)} - {ɛ_{de}\left( \lambda_{2} \right)}} \right) - {\frac{\mu_{a}\left( \lambda_{2} \right)}{\mu_{a}\left( \lambda_{1} \right)} \cdot \left( {{ɛ_{ox}\left( \lambda_{1} \right)} - {ɛ_{de}\left( \lambda_{1} \right)}} \right)}}} & (5)\end{matrix}$

Performing such processing at each position yields the oxygen saturationat each position, so an oxygen saturation distribution can be obtained.FIG. 3 illustrates an example of a display screen where an oxygensaturation distribution has been obtained from the absorptioncoefficient distribution of wavelength λ₁ and the absorption coefficientdistribution of wavelength λ₂. Reference numeral 301 denotes the oxygensaturation distribution at wavelength λ₁, reference numeral 302 denotesthe oxygen saturation distribution at wavelength λ₂, and referencenumeral 303 denotes an image of the oxygen saturation distribution. Theoxygen saturation distribution may be three-dimensional distributiondata (set data of voxels) corresponding to a certain region within thesubject, or may be two-dimensional distribution data (set data ofpixels) corresponding to one cross-section thereof.

The statistical information obtaining unit 6 obtains statisticalinformation indicating variance in at least part of the distribution inthe oxygen saturation distribution obtained by the second distributionobtaining unit 5. It is sufficient for the statistical informationindicating variance to be an evaluation index indicating variance invalue within a distribution (particularly variance from a center value(typically an average value)), such as dispersion value, standarddeviation, half value of histogram, entropy (average information amountof pixel values), and so forth. Also, in a case of an evaluation indexwhere the larger the value is, the smaller the variance is, calculationprocessing may be performed such as integration of the evaluation indexby −1, so that the evaluation index is read as having smaller variancethe smaller the value is. A specific way of obtaining statisticalinformation will be described later with reference to FIG. 4.

The positional deviation between the absorption coefficientdistributions obtained for each wavelength (i.e., the positionaldeviation between the probe and subject at each wavelength) can beestimated by this statistical information in the present embodiment.Specifically, in a case where this statistical information (e.g.,dispersion value) is small, the positional deviation among wavelengthscan be determined to be small, and if the statistical information islarge, and the positional deviation among wavelengths is large. Notethat “positional deviation between probe and subject” includes“positional deviation between probe and partial region within subject”.

The reason why positional deviation can be determined from statisticalinformation indicating dispersion in oxygen saturation distribution inthis way will be described. Generally, the oxygen saturation value ofblood in blood vessels differ at micro-level viewpoints such as at bloodcells or clusters thereof. However, when observing blood cells (10 μm orsmaller) in blood vessels with a device having spatial resolution ofseveral hundred μm to several mm (in a range of 100 μm to 10 mm), theoxygen saturation of blood cells and clusters appear to be spatiallyaveraged. Also, even when using a device having high spatial resolution(10 μm or smaller) but with poor temporal resolution, the oxygensaturation of blood cells and clusters appear to be spatially averaged.That is to say, when the device being used has spatial resolution ortemporal resolution such that the oxygen saturation of individual bloodcells cannot be distinguished, the oxygen saturation of blood in theblood vessels becomes an averaged value.

This is generally true for large blood vessels around several hundred μmto several mm, but is not restricted to this, and the same can beconceived to be true at blood vessel portions where there is no tissueother than blood vessel partway along the blood vessel and blood refluxspeed is fast. This sort of phenomenon is thought to occur regardless ofdifferences between arteries and veins.

Accordingly, if there is no deviation between absorption coefficientdistributions, and the oxygen saturation distribution has been correctlyfound, the value of statistical information such as the dispersion valueof oxygen saturation distribution will be contained in a relativelysmall value. Accordingly, in a case where the statistical informationrepresenting dispersion assume a large value, it is conceivable thatvariance within the oxygen saturation distribution is large, due todeviation among the absorption coefficient distributions.

The region setting unit 13 can set an object region to obtain an oxygensaturation distribution within an absorption coefficient distribution,set a target region for obtaining statistical information within anoxygen saturation distribution, or the like. That is to say, the oxygensaturation distribution in the present embodiment may be obtained for apartial region within the absorption coefficient distribution, and isnot restricted to cases of the same range as the absorption coefficientdistribution. The region setting unit 13 further may set a target regionfor further obtaining statistical information with regard to an oxygensaturation distribution within the object region obtained by the seconddistribution obtaining unit 5. This is because there is no need to useoxygen saturation distribution at locations other than blood asstatistical information.

As for a method for the region setting unit 13 to set object regions andtarget regions, there is a method of performing settings based on regioninformation which the user has input using the input unit 12. There isalso a method where the region setting unit 13 performs settingsautomatically, based on the intensity within the initial sound pressuredistribution or absorption coefficient distribution. Either atwo-dimensional region or a three-dimensional region may be set. Theshape of the region being set may be a shape matching the structuralpart, or a cuboid, or any shape.

As for a method where the region is set based on user input, there is amethod where the region setting unit 13 sets the region based on aregion which the user has set on the image using the input unit 12 whileviewing a display screen such as shown in FIG. 3. For example, in a caseof setting an object region, the user selects (surrounds as a settingrange) part or all of a structural part conceivably a blood vessel, onan image from one of the absorption coefficient distributions of λ₁ andλ₂. The region setting unit 13 may set this setting range as an objectrange for obtaining the oxygen saturation distribution. Further, atarget range may be set by the user selecting a partial region on theimage of the oxygen saturation distribution. The region setting unit 13preferably performs this setting by surrounding a regiontwo-dimensionally if the distribution is two-dimensional, andthree-dimensionally if the distribution is three-dimensional.

In a case where the region setting unit 13 automatically sets theregion, first, processing is performed to extract an object region suchas a blood vessel or the like in a region in the same range as theabsorption coefficient distribution, and thereafter at least a part ofthe region within the object region is preferably set as a targetregion. As for a method of setting the object region, there is a methodof extracting a portion where the intensity is higher than apredetermined value (threshold value) in the absorption coefficientdistribution, and so forth. There are also other methods which may beused, such as a method of performing template matching processing usingan image template unique to the object structure part, or methods ofextracting by performing image processing using snakes, Level Set, orthe like. If the S/N ratio of the object structure part portion is highin the distribution, the entire structure body does not have to beextracted. As for a method for setting the target region, scanning canbe performed from the edge of the oxygen saturation distribution withinthe object region set earlier, and labeling processing performed onportions where the structure part is found to exist as a singlecontinuation, thereby setting the target region.

The shift unit 7 performs coordinate shift processing for relativelyshifting the coordinates among the absorption coefficient distributionsused for oxygen saturation distribution calculation, for the seconddistribution obtaining unit 5. This coordinate shift processing can beperformed by copying the absorption coefficient distribution within theset target region, and replacing with the absorption coefficientdistribution at the shift destination. Note however, that alpha blendingwith the absorption coefficient distribution at the shift destinationmay be performed at a suitable rate. Alternatively, affine transform, ornon-rigid transform or the like by performing optimization calculationof statistical information as evaluation functions, may be performedinstead of replacing distributions or performing alpha blending. At thetime of performing optimization calculation, a gradient algorithm may beused such as in method of steepest descent or conjugate gradient method.The specific shift method will be described later with reference to FIG.6. The shift unit 7 performs this relative coordinate shift processingseveral times. The second distribution obtaining unit 5 obtains theoxygen saturation distribution for each shift, and the statisticalinformation obtaining unit 6 obtains statistical information for thetarget region for each oxygen saturation distribution obtained. That isto say, the second distribution obtaining unit 5 obtains theconcentration-related distribution within the target region multipletimes for multiple times of coordinate shift processing, and thestatistical information obtaining unit 6 obtains a plurality ofstatistical information.

The deciding unit 14 decides the amount of positional deviation, basedon the multiple statistical information obtained by multiple times ofcoordinate shift processing. For example, the vector in coordinate shiftin a case where the dispersion value of the oxygen saturationdistribution within the target region is smallest is decided to be thepositional deviation among absorption coefficient distributions. That isto say, the absorption coefficient distributions in relative positionalrelation in a case where the dispersion value is the smallest, becomesthe absorption coefficient distributions after correction of positionaldeviation. A specific deciding method will be described later withreference to FIG. 6.

It should be noted, however, that positional deviation may be decided bythe deciding unit 14 based on coordinate shift in a case where thestatistical information is smaller than a predetermined value, and notjust a case of the statistical information being the smallest value.That is to say, even if not the smallest value, of a value which can betolerated as variation is set as a threshold value and the statisticalinformation is smaller than that threshold value, the vector ofcoordinate shift at that time can be decided as the positionaldeviation. Such settings also enable oxygen saturation distributionswith suppressed (reduced) effects of positional deviation to beobtained.

In a case where the statistical information is smaller than statisticalinformation in a case with no coordinate shift, out of the multiplestatistical information, the relative positional relation between theabsorption coefficient distributions is in a state where positionaldeviation has been suppressed (reduced). Accordingly, the predeterminedvalue may be a statistical information value in a case of no performingcoordinate shifting. In the present embodiment, it is sufficient ifpositional deviation is suppressed, even if not completely corrected.That is to say, even if the effects of positional deviation are notcompletely eradicated by coordinate shifting, an advantage can beobtained that oxygen saturation distribution with higher accuracy thanthe related art can be obtained as long as the effects of positionaldeviation are reduced.

The display control unit 10 generates image data of displaying on thedisplay unit 8, based on the distribution data such as the absorptioncoefficient distribution generated by the first distribution obtainingunit 4 and the oxygen saturation distribution generated by the seconddistribution obtaining unit 5. Specifically, image processing such asluminance conversion, distortion correction, logarithmic compression,and so forth, is performed based on the distribution data. Further,display control is performed such as arraying various types of displayitems along with the distribution data, updating the display based oninstructions from the input unit 12, and so forth.

The control unit 11 has the region setting unit 13, statisticalinformation obtaining unit 6, shift unit 7, and the deciding unit 14,and also supplies control signals and data necessary for theconfiguration blocks within the photoacoustic apparatus, to controlthese configuration blocks. Specifically, the control unit 11 suppliessignals instructing the light source 1 to emit light, and controlsignals to the converting elements 3 within the probe 30. The controlunit 11 also controls signal amplifying at the signal collecting unit 9,control of AD conversion timing, control of storage of received signals,and so forth. The control unit 11 also transmits control signals relatedto distribution data generation to the first and second distributionobtaining units, and reception of distribution data. The control unit 11also transmits control signals relating to image generating to thedisplay control unit 10, and receives image data from the displaycontrol unit 10. The control unit 11 further is connected to the inputunit 12 for the user to input various types of operations andinstructions, and accepts input information from the user via the inputunit 12. In a case of moving the probe 30, the control unit 11 mayperform moving control. Moreover, the control unit 11 may hold receptionsignals, generated distribution data, display image data, various typesof measurement parameters, and so forth.

Processing Flow at Signal Processing Unit 40

Next, the processing flow at the signal processing unit 40 will bedescried. FIG. 2 is a flowchart illustrating the processing flow at thesignal processing unit 40 according to the present embodiment. Note thatthe flow in FIG. 2 starts from a state where reception signals aresequentially input to the signal collecting unit 9 within the signalprocessing unit 40 from the probe, for each wavelength of theirradiation light, and processing such as AD conversion and amplifyingand so forth has been performed at the signal collecting unit 9.

In step S101, the first distribution obtaining unit 4 obtains theabsorption coefficient distribution at the wavelength λ₁ using receptionsignals of light with the wavelength λ₁, and obtains the absorptioncoefficient distribution at the wavelength λ₂ using reception signals oflight with the wavelength λ₂.

In step S102, the second distribution obtaining unit 5 generates oxygensaturation distribution within the object region using the absorptioncoefficient distribution at wavelength λ₁ and the absorption coefficientdistribution at wavelength λ₂. Note that the object region may be apartial region within the absorption coefficient distribution set by theregion setting unit 13, or the object region may be the same range asthe range of the absorption coefficient distribution.

In step S103, the region setting unit 13 sets at least part of theoxygen saturation distribution obtained by the second distributionobtaining unit 5 as the target region. Here, an example will bedescribed in which the target region is set based on user input. In thiscase, the display control unit 10 generates image data based on data ofthe oxygen saturation distribution obtained by the second distributionobtaining unit 5. The display control unit 10 displays an image ofoxygen saturation distribution such as illustrated in FIG. 3, on thedisplay unit 8. Upon the user using the input unit 12 to instruct apredetermined region on the image of the displayed oxygen saturationdistribution, the region setting unit 13 sets the specified region asthe target region.

In step S104, the statistical information obtaining unit 6 obtainsstatistical information indicating the variance in the oxygen saturationdistribution within the target region. An example of obtainingdispersion as statistical information will be described. Dispersion canbe expressed as in the following Expression (6).

$\begin{matrix}{\sigma^{2} = {\frac{1}{m}{\sum\limits_{i = 1}^{m}\;\left( {M - x_{i}} \right)^{2}}}} & (6)\end{matrix}$

When M represents the average value of a population of m pieces of data(x₁ through x_(m)), the arithmetic mean σ² represents dispersion inExpression (6). Note that in the present embodiment, x_(i) correspondsto each voxel value (or pixel value), and M corresponds to the averagevalue thereof.

Description will be made in further detail with reference to FIG. 4.FIG. 4 is a schematic diagram illustrating 4×4 distribution for a targetregion. Note that the Z axis is not drawn in FIG. 4 for the sake ofsimplicity, and only the X-Y coordinates are shown for description. Thevalues described in each voxel in FIG. 4 represent the intensity of thatvoxel. Here, the average value M=(total of elements)/16=7.25, and thedispersion value σ²=Σ(elements−7.25)²/16=19.19. The dispersion valueobtaining in this way is saved in memory in the control unit 11.

In step S105, the shift unit 7 performs processing for relativepositional shifting of the absorption coefficient distribution atwavelength λ₁, and the absorption coefficient distribution at wavelengthλ₂. FIG. 6 is a schematic diagram for describing a method for obtainingcoordinate shift and dispersion. In FIG. 6, the distribution at the leftside represents the absorption coefficient distribution at wavelengthλ₁, the distribution at the right side represents the absorptioncoefficient distribution at wavelength λ₂, both being represented as aset of 6×6 voxels. Note that the Z axis is not drawn here for the sakeof simplicity, and only the X-Y coordinates are shown for description.The values described in each voxel represent the intensity of that voxel(value of absorption coefficient in this case).

Now, the position of a 4×4 distribution “a” at the center of theabsorption coefficient distribution for the wavelength λ₁ represents theregion set as the target region in S103. The shift unit 7 performsparallel movement of the absorption coefficient distribution atwavelength λ₂ in one of the depth, width, and height directions (X, Y, Zdirections) by one voxel at a time (by one coordinate at a time), withthe absorption coefficient distribution of the target region of thiswavelength λ₁ as a reference.

FIG. 6 illustrates a distribution moved −1 in the X direction and +1 inthe Y direction from a reference position, which is a distribution b(0,0) of wavelength λ₂ at the same position as the target region(distribution “a”) of the wavelength λ₁, as b(−1, 1). Shifting −1 and +1each in the X and Y directions gives a total of nine patterns forcoordinate shifting.

In step S106, the second distribution obtaining unit 5 obtains theoxygen saturation distribution of the target region, using theabsorption coefficient distributions of the wavelength λ₁ and wavelengthλ₂ relatively coordinate-shifted. For example, FIG. 6 yields the oxygensaturation distribution using the target region of the wavelength λ₁(distribution “a”) and the distribution b(−1, 1) of the wavelength λ₂after coordinate shifting.

In step S107, the statistical information obtaining unit 6 obtainsstatistical information representing variance of oxygen saturationdistribution within the target region obtained in S106. The distributionof dispersion, which is the statistical information, is illustrated atthe bottom of FIG. 6. For example, the dispersion of the oxygensaturation distribution obtained using the target region of thewavelength λ₁ (distribution “a”) and the distribution b(−1, 1) of thewavelength λ₂ after coordinate shifting is shown as dispersion c(−1,−1). The dispersion value obtained as statistical information in thisstep is saved in memory in the control unit 11, in the same way as inS104.

In S108, the control unit 11 determines whether the coordinate shiftingby the shift unit 7 has all ended. For example, upon all nine patterns,described above regarding FIG. 6, of the coordinate shifting ending,determination is made that the coordinate shifting has ended. Generally,in a case of an apparatus which receives photoacoustic waves in a stillstate, the amount of positional deviation that occurs is within 2 mm orso. Accordingly, in a case of moving in parallel a maximum of 8 voxelsin the X direction, Y direction, and Z direction, the number ofcoordinate shift patterns is 4,913. In detail, there are 8+1+8=17combinations in the X direction (8 in the positive direction, 1 notshifting (staying at X=0), and 8 in the negative direction). The Ycoordinates and Z coordinate also have these combinations, so the totalnumber of combination patterns for coordinate shifting is(8+1+8)×(8+1+8)×(8+1+8)=4,913 patterns.

Thus, upon all of the combination patterns set beforehand ending, thecoordinate shifting is determined to have ended. In a case where thecoordinate shifting is determined to have ended, the flow advances toS109, and if the coordinate shifting has not ended, the flow returns toS105.

Thus, repeating S105 through S108 obtains multiple oxygen saturationdistributions based on the target region of wavelength λ₁ (distribution“a”) and the distribution b(i, j) of the wavelength λ₂. Further,dispersion corresponding to each of the multiple oxygen saturationdistributions is also obtained.

In S109, the deciding unit 14 decides the positional deviation betweenthe probe and the subject, based on the plurality of statisticalinformation calculated in S104 and S107. The coordinate shift where thedispersion is the smallest out of the multiple dispersions is decided asthe positional deviation between the absorption coefficientdistributions (relative positional deviation between the absorptioncoefficient distribution of the wavelength λ₁ and the absorptioncoefficient distribution of the wavelength λ₂). The reason is that thesmallest value for dispersion can be though to mean that the variancewithin the obtained oxygen saturation distribution is the smallest.

That is to say, the vector of the coordinate shift where the dispersionis the smallest value corresponds to the relative positional deviationbetween the probe and the subject. In other words, the relativepositional relation between the absorption coefficient distributionswhen the dispersion is the smallest is the absorption coefficientdistributions with each other after positional deviation correction. Ofthe nine patterns of coordinate shift in FIG. 6, the dispersion(represented by c(−1, −1)) at the time of coordinate shift representedby distribution b(−1, 1) of wavelength λ₂ is the smallest dispersion.The deciding unit 14 transmits the vector of the coordinate shift thusdecided (information relating to the positional deviation) to the seconddistribution obtaining unit 5.

In S110, the second distribution obtaining unit 5 outputs oxygensaturation distribution data obtained using the absorption coefficientdistributions after positional deviation correction to the displaycontrol unit 10, based on information relating to positional deviationfrom the deciding unit 14. Note that oxygen saturation distribution isobtained for the multiple times of coordinate shifting, so the seconddistribution obtaining unit 5 can read out the oxygen saturationdistributions from the memory and transmit to the display control unit10, if the oxygen saturation distributions have been saved in the memoryin the control unit 11. The second distribution obtaining unit 5 maynewly obtain an oxygen saturation distribution using the absorptioncoefficient distributions following positional deviation correction.

In S111, the display control unit 10 generates image data based on theoxygen saturation distribution data output from the second distributionobtaining unit 5, and displays on the display unit 8.

Thus, according to the present embodiment, positional deviation isobtained based on oxygen saturation distributions for imaging, so moreaccurate positional deviation detection can be realized, and oxygensaturation distribution can be obtained with higher accuracy.

Note that the second distribution obtaining unit 5 in theabove-described example obtains oxygen saturation distribution as theconcentration-related distribution, but the present embodiment is notrestricted to this. As described earlier, “distribution of valuesrelating to concentration of substance (concentration-relateddistribution) obtained using “property distribution based on opticalabsorption” of multiple wavelengths, suffices. That is to say,distributions such as “weighed oxygen saturation value”, “totalhemoglobin concentration”, “oxyhemoglobin concentration”,“deoxyhemoglobin concentration”, “glucose concentration”, “collagenconcentration”, “melanin concentration”, “volume fraction” of fat andwater, and so forth are all acceptable.

The first distribution obtaining unit 4 has been described in the aboveexample as obtaining absorption coefficient distributions as propertydistributions based on optical absorption, but the present embodiment isnot restricted to this, and may use “sound pressure distribution(typically initial sound pressure distribution)” or “optical energyabsorption density distribution”. For example, since μ_(a) can beexpressed as P/(Γ·ϕ) from Expression (1), substituting P/(Γ·ϕ) for μ_(a)in Expression (5) enables the oxygen saturation distribution to bedirectly obtained from the initial sound pressure. That is to say, thesecond distribution obtaining unit 5 can directly obtain the oxygensaturation distribution from the initial sound pressure distributiondata, without the first distribution acquiring unit 4 having to gothrough the process of obtaining absorption coefficient distributionafter obtaining the initial sound pressure distribution.

Also, while an example has been described above where the structure bodyserving as the object region is the blood portion within blood vessels,the present embodiment is not restricted to this. Blood vessel walls,lymph ducts, muscle tissue, mammary gland tissue, fat tissue, andaggregations of substances externally injected such as a moleculartarget drug serving as a radiopaque dye, may be used. The object regionis preferably selected according to the substance regarding which theconcentration is to be obtained.

Specific configuration examples of the components of the presentembodiment will be described next.

Light Source 1

The light source 1 preferably is a pulsed light source capable ofemitting pulsed light of pulses in the nanosecond to microsecond order.The specific pulse width used is in the range of around 1 nanosecond to100 nanoseconds. The wavelength used is in the range of 400 nm to 1600nm. Particularly, in cases of imaging deep portions of a living body,light of a wavelength band referred to as a “biological window” due tolow absorption by background tissue of the body is used. Specifically,light of a wavelength range of 700 nm to 1100 nm is preferable. On theother hand, the visible light region is preferably used when imagingblood vessels near the surface of the body at high resolution. However,terahertz waves, microwaves, and radio wave regions may also be used.

Specifically, laser is preferably for the light source 1, and laser withvariable oscillation wavelength is preferable since the presentembodiment uses light of multiple wavelengths. However, an arrangementmay be made wherein multiple laser devices with different oscillationwavelengths are used, switching oscillation from one to another, sinceit is sufficient to be able to irradiate the subject with multiplewavelengths. Also, an arrangement may be made wherein multiple laserdevices with different oscillation wavelengths are used, switchingemission from one to another. Arrangements where multiple laser devicesare used will also be collectively referred to as “light source”.

Examples of lasers which can be used include solid-state laser, gaslaser, dye laser, semiconductor laser, and so forth. Particularlypreferable are pulsed lasers such as neodymium-doped yttrium aluminumgarnet (Nd:YAG) laser and alexandrite laser. Also usable are titaniumsapphire (Ti:sa) laser which uses Nd:YAG laser light as excitationlight, and optical parametric oscillator (OPO) laser. Light-emittingdiodes or the like may also be used instead of laser.

The pulsed laser output from the light source is preferably guided tothe subject by members which propagate light (optical members), such asoptical fiber, lenses, mirrors, diffusing plates, and so forth. The spotshape and beam density of the pulsed light can be changed using suchoptical members when guiding the pulsed light.

Probe 30

The probe 30 has one or more converting elements 3. Any sort ofconverting element may be used for the converting elements 3 as long asphotoacoustic waves can be received and converted into electric signals,examples including piezoelectric elements using the piezoelectricphenomenon, such as lead zirconate titanate (PZT), converting elementsusing light resonance, electrostatic capacitance type convertingelements such as capacitive micromachined ultrasonic transducers (CMUT),and so forth. In a case of having multiple converting elements 3, theconverting elements 3 are preferably arrayed on a plane or curvedsurface in an array called 1 D array, 1.5 D array, 1.75 D array, 2 Darray, or the like.

The probe 30 may be configured to mechanically move in relation to thesubject, and may be a hand-held type probe 30 where the user holds andmoves the probe 30. In a case of a photoacoustic microscope, the probe30 is preferable a focusing probe, with the probe 30 mechanically movingover the surface in relation to the subject. The irradiation position ofthe irradiation light and the probe 30 preferably move synchronously. Anamplifier for amplifying analog signals output from the convertingelements 3 may be provided within the probe 30.

Input Unit 12

A mouse, keyboard, touch panel, audio input unit, or the like, may beused as the input unit 12. The input unit 12 is not restricted to beinga configuration which the photoacoustic apparatus according to thepresent embodiment has, and may be provided separately and connected tothe photoacoustic apparatus.

Display Unit 8

A liquid crystal display (LCD), cathode ray tube (CRT), organicelectroluminescence (EL) display, or the like, may be used as thedisplay unit 8. The display unit 8 is not restricted to being aconfiguration which the photoacoustic apparatus according to the presentembodiment has, and may be provided separately and connected to thephotoacoustic apparatus.

Signal Processing Unit 40

A circuit generally called a data acquisition system (DAS) may be suedfor the signal collecting unit 9. More specifically, the signalcollecting unit 9 includes an amplifier for amplifying received signals,an AD converter for digitizing analog received signals, afirst-in-first-out (FIFO) buffer for storing received signals, randomaccess memory (RAM), and other such memory, and so forth.

A central processing unit (CPU), microprocessor unit (MPU), graphicsprocessing unit (GPU), or some other like processor may be used as thefirst distribution obtaining unit 4 and second distribution obtainingunit 5. A computing circuit such as a field programmable gate array(FPGA) chip or the like may be used as well. The first distributionobtaining unit 4 and second distribution obtaining unit 5 are notrestricted to being configured using a single processor or computingcircuit, and may be configured using multiple processors or computingcircuits.

The first distribution obtaining unit 4 and second distributionobtaining unit 5 may have memory to store reception signals output fromthe signal collecting unit 9. The memory typically is configured as readonly memory (ROM), RAM, a hard disk, and other like storage media. Thememory is not restricted to being configured using a single storagemedium, and may be configured using multiple storage media.

The display control unit 10, shift unit 7, statistical informationobtaining unit 6, region setting unit 13, deciding unit 14, and controlunit 11 may also in like manner be configured using one of or acombination of processors such as a CPU or GPU or the like, and circuitssuch as FPGA chips or the like. These components may also have memory tostore reception signals, distribution data, display image data, varioustypes of measurement parameters, and so forth. The memory typically isconfigured as one or more of read only memory (ROM), RAM, a hard disk,and other like storage media.

FIG. 5 is a schematic diagram illustrating the relationship between aspecific example of the signal processing unit 40 and external devices.In the example in FIG. 5, the signal processing unit 40 includes a DAS201, memory 202, a CPU 203, and a GPU 204.

The DAS 201 handles a function of the signal collecting unit 9 in thepresent embodiment. Digital signals transferred from the DAS 201 arestored in the memory 202.

The CPU 203 functions as part of the first distribution obtaining unit4, second distribution obtaining unit 5, display control unit 10, shiftunit 7, statistical information obtaining unit 6, region setting unit13, deciding unit 14, and control unit 11 according to the presentembodiment. Specifically, the CPU 203 accepts various types ofparameters and instructions relating to operations from the user via theinput unit 12, generates necessary control information, and controls thefunctional blocks via a system bus 200. The CPU 203 also can performsignal processing such as integration processing and correctionprocessing of digital signals stored in the memory 202. The CPU 203further writes the digital signals after signal processing into thememory 202 again, to be used for the GPU 204 to generate distributiondata.

The GPU 204 functions as part of the first distribution obtaining unit4, second distribution obtaining unit 5, display control unit 10, shiftunit 7, statistical information obtaining unit 6, region setting unit13, deciding unit 14, and control unit 11 according to the presentembodiment. Specifically, the GPU 204 creates distribution data usingdigital signals that have been subjected to signals processing by theCPU 203 and written to the memory 202. The GPU 204 also can create imagedata by subjecting the created distribution data to various types ofimage processing, such as luminance conversion, distortion correction,cropping of target region, and so forth. The CPU 203 is also capable ofsimilar processing.

First Example

A more specific example will be described will be described withreference to FIG. 1. In the present example, the subject is a breast,the subject is irradiated by light across a holding member formed ofpolymethylpentene which holds the subject, and the probe 30 receives thephotoacoustic waves across the holding member. The probe 30 is a 2 Darray probe having multiple converting elements in the frequency band of1 MHz±40%.

First, the subject is irradiated by pulse light from the light source 1having a wavelength of 797 nm in the present example, and the probe 30receives the photoacoustic waves. The signal processing unit 40 of theconfiguration illustrated in FIG. 5 performs image reconfiguration usinguniversal back projection based on the reception signals that have beenreceived. An absorption coefficient distribution is then created usingthe obtained initial sound pressure distribution, light quantitydistribution, and the Grueneisen constant. The values of the absorptioncoefficient distribution are in voxel data, each voxel being a cube 0.25mm in each dimension. The obtained absorption coefficient distributionis 185 voxels deep, 481 voxels wide, and 281 voxels high.

Next, in order to generate positional deviation, the subject is shiftedby 1 mm. Thereafter, the subject is irradiated by pulse light from thelight source 1 having a wavelength of 756 nm, and the probe 30 receivesthe photoacoustic waves. The signal processing unit 40 performs imagereconfiguration using universal back projection based on the receptionsignals that have been received. The signal processing unit 40 thencreates an absorption coefficient distribution using the obtainedinitial sound pressure distribution, light quantity distribution, andthe Grueneisen constant.

Now, FIG. 7 illustrates the way in which positional deviation occursamong the absorption coefficient distributions between the wavelengths.The peak to the right side in FIG. 7 is the signal intensity at acertain cross-sectional position in the absorption coefficientdistribution for 756 nm, and the peak to the left side in FIG. 7 is thesignal intensity at the same cross-sectional position in the absorptioncoefficient distribution for 797 nm. It can be seen from FIG. 7 that theposition of the peak to the right is shifted as to the peak at the left.

The signal processing unit 40 calculates the oxygen saturationdistribution based on the absorption coefficient distribution for 756 nmand the absorption coefficient distribution for 797 nm. Calculation ofthe oxygen saturation is performed among corresponding voxels in thedifferent wavelengths, with the oxygen saturation distribution being 185voxels deep, 481 voxels wide, and 281 voxels high, the same as with theabsorption coefficient distributions. A positional deviation state isgenerated between the subject and the probe, so this oxygen saturationdistribution has not been appropriately calculated. FIG. 8 illustratessignal intensity of oxygen saturation at a certain cross-section in theoxygen saturation distribution. The oxygen saturation has not beenproperly calculated, so it can be seen from FIG. 8 that there isvariance in values at the region thought to be corresponding to theblood portion within the oxygen saturation distribution. Assumption willbe made that the oxygen saturation distribution is in a range of 0% to100%. Now, FIG. 9 illustrates an ideal case where there is no positionaldeviation between absorption coefficient distributions for eachwavelength. It can be seen that there is no variance in oxygensaturation in FIG. 9, since the oxygen saturation corresponding to theblood portion of the oxygen saturation distribution has been calculatedproperly.

Next, upon receiving input from the user, the signal processing unit 40sets a partial three-dimensional region of the calculated oxygensaturation distribution as a target region. The target region has beenset at a portion where intensity is high in the 797 nm initial soundpressure distribution corresponding to the blood portion. An oxygensaturation histogram is calculated in the set target region, anddispersion is calculated from the calculate histogram. The value ofdispersion obtained here is high, due to the effects of positionaldeviation of the subject.

Next, using the 797 nm absorption coefficient distribution as areference, the target region within the 756 nm absorption coefficientdistribution is shifted in parallel one voxel each in the depth, width,and height directions, up to eight voxels. The oxygen saturationdistribution within the target region and the dispersion of oxygensaturation distribution are calculated in each of the shifting patterns.The total number of shifting patterns is (8+1+8)×(8+1+8)×(8+1+8)=4,913patterns, including not shifting.

The shifting pattern of which the dispersion value has the smallestvalue out of the 4,913 patterns of dispersion values calculated asdescribed above is taken as the vector of the coordinate shifting. Thesignal processing unit 40 then performs coordinate shifting of the 756nm absorption coefficient distribution based on the obtained coordinateshift vector, and thus obtains the oxygen saturation distribution.Accordingly, an oxygen saturation distribution regarding whichpositional deviation correction among waveforms has been corrected canbe obtained in the present example. An advantage thereof is thatrelative positional deviation among many waveforms can be corrected fromthe initial sound pressure distribution and absorption coefficientdistribution obtained at the photoacoustic apparatus, without having touse an ultrasound apparatus, and calculation accuracy of oxygensaturation is improved.

Second Embodiment

Next, a second embodiment will be described. The photoacoustic apparatusaccording to the present embodiment uses the same device configurationas the photoacoustic apparatus according to the first embodiment, sodetailed description of the configurations will be omitted. Notehowever, there is difference as to the first embodiment with regard tothe processing performed by the signal processing unit 40, sodescription will be made below focusing on differences as to the firstembodiment.

A feature of the photoacoustic apparatus according to the presentembodiment is that the region setting unit 13 sets the target regionautomatically. In the steps in FIG. 2, the same processing as in thefirst embodiment is performed up to S102. In S103 in the presentembodiment, the region setting unit 13 first performs masking of theblood vessel portion in the oxygen saturation distribution. The regionsetting unit 13 sets to 0 the intensity at positions in the oxygensaturation distribution corresponding to positions where the intensityin the absorption coefficient distribution of one wavelength is lowerthan a predetermined value. The region setting unit 13 then sets atleast a partial region that continuous out of the region where theintensity is a predetermined value or higher. Thus, the region settingunit 13 can set the blood vessel portion (portions where intensity inthe absorption coefficient distribution is high) as a target region.Initial sound pressure distribution may be used for masking instead ofthe absorption coefficient distribution.

The processing of S104 and thereafter is the same as in the firstembodiment. The region setting unit 13 sets the target regionautomatically in the present embodiment, based on the intensity in afeature distribution, based on optical absorption in absorptioncoefficient distribution or the like, so the user is spared this work,and an apparatus with better ease-of-use is provided.

Third Embodiment

Next, a third embodiment will be described. The photoacoustic apparatusaccording to the present embodiment uses the same device configurationas the photoacoustic apparatus according to the first and secondembodiments, so detailed description of the configurations will beomitted. Note however, there is difference as to the first and secondembodiments with regard to the processing performed by the signalprocessing unit 40, so description will be made below focusing on thedifferences.

A feature of the photoacoustic apparatus according to the presentembodiment is that the region setting unit 13 sets multiple targetregions at positions different from each other. In the steps in FIG. 2,the same processing as in the first embodiment is performed up to S102.In S103 in the present embodiment, the region setting unit 13 firstperforms binarization in the oxygen saturation distribution, in the sameway as in the second embodiment. Initial sound pressure distribution maybe used for binarization instead of the absorption coefficientdistribution.

In a case where a region corresponding to the blood vessel portion inthe oxygen saturation distribution is large, there is a possibility thatpositional deviation vectors (positional deviation amount and positionaldeviation direction) may differ within that region. Accordingly, in thepresent embodiment, the region setting unit 13 sets multiple targetregions in the oxygen saturation distribution obtained by binarization.Note that each target region preferably is in a region corresponding tothe blood vessel portion. The size of each target region is a size setin the device beforehand, such as a 20×20 pixel square iftwo-dimensional, or a 20×20×20 voxel cube if three-dimensional, or thelike. The size may be settable by the user.

Thereafter, the processing of S104 and thereafter is applied to each ofthe multiple target regions. Note that in a case where relativecoordinate sift is performed among the absorption coefficientdistributions in each target region, there may be discrepancy betweenthe absorption coefficient distribution in one target region and theabsorption coefficient distribution in an adjacent target region. Thisis because there are cases where the positional deviation between theprobe 30 and the subject is different in each target region. In thiscase, the region setting unit 13 preferably sets the target regions sothat the target regions have overlapping portions. Alternatively, afterending one target region up to S109, an adjacent target region may beset so that there is no overlapping with the one target region and nodiscrepancy. That is to say, the flow from setting target regionsthrough deciding positional deviation may be repeated while changing thetarget regions the processing of S110 and thereafter is the same as inthe first and second embodiments.

As described above, the region setting unit 13 according to the presentembodiment sets multiple target regions, so positional deviationcorrection can be performed more accurately. While the region settingunit 13 automatically sets the target regions in the above-describedexample, an arrangement may be made where the region setting unit 13sets multiple target regions based on user input, as in the firstembodiment.

Fourth Embodiment

Next, a fourth embodiment will be described. The photoacoustic apparatusaccording to the present embodiment is illustrated in FIG. 10. Adeformation unit 17 is a part which differs from the photoacousticapparatus according to the first through third embodiments. Other thanthat, the photoacoustic apparatus according to the present embodimentuses the same device configuration as the photoacoustic apparatusaccording to the first through third embodiments, so detaileddescription of the configurations will be omitted. Note however, thereis difference as to the first through third embodiments with regard tothe processing performed by the signal processing unit 40, sodescription will be made below focusing on the differences.

The photoacoustic apparatus according to the present embodiment is asubject information obtaining apparatus that sets conditions ofconstraint on the positional deviation among images from multiple imagesobtained for each wavelength, and performs affine transform, non-rigidtransform, and like deformation processing, so that the dispersion valueof oxygen saturation is the smallest, thereby performing accuratedeformation positioning.

An absorption coefficient distribution of the subject is created foreach of 756 nm and 797 nm light, in the same way as with the firstembodiment. positional deviation correction is performed between theabsorption coefficient distributions of the two wavelengths. First, inthe steps in FIG. 11, processing the same as in the first embodiment isperformed up to S202. In S203 the dispersion value of oxygen saturationis calculated for the entire region of the oxygen saturationdistribution in the present embodiment. The oxygen saturation value isweighted according to the intensity of the absorption coefficientdistribution at this time.

Thereafter, in S204, the deformation unit 17 performs deformation of the756 nm absorption coefficient distribution. Known methods based onsimilarity of luminance information may be used as the method fordeformation. For example, grid-like points that correspond betweenpixels are set on the 756 nm absorption coefficient distribution, andthe positions of the corresponding points on the 797 nm absorptioncoefficient distribution are estimated using similarity of luminanceinformation. This processing may utilize free form deformation (FFD),described in “T.W. Sederberg ‘Free Form Deformation of solid geometricmodels,’ Proc. SIGGRAPH '86, vol. 20, no. 4, pp. 151-160, 1986”, forexample. At this time, the conditional optimization problem whichminimizes the cost function E in Expression (7) has to be solved as theevaluation method for deformation positioning.E(I,T)=(1−ZNCC(I,T))+λf(I,T)  (7)

The first term on the right side in Expression (7) is a cost functionaccording to a normalized cross-correlation coefficient, and the secondterm f(I, T) is sometimes called a constraint term or a penalty term.The second term is a term which applies a constraint to the solution ofthe normalized cross-correlation cost function of the first term, so asto narrow down to a more suitable solution (regularization term). λ isan optional constant to balance the least square term and the constraintterm, and is a value determined by experience. I and T are distributionwithin the target region of the 756 nm and 797 nm absorption coefficientdistributions, respectively. The following Expression (8) yields anormalized cross-correlation (zero mean normalized cross-correlation(ZNCC)).

$\begin{matrix}{{{ZNCC}\left( {I,T} \right)} = \frac{\sum\limits_{k = 0}^{N - 1}\;{\sum\limits_{j = 0}^{M - 1}\;{\sum\limits_{i = 0}^{L - 1}\;\left( {\left( {{I\left( {i,j,k} \right)} - \overset{\_}{I}} \right)\left( {{T\left( {i,j,k} \right)} - \overset{\_}{T}} \right)} \right)}}}{\sqrt{\sum\limits_{k = 0}^{N - 1}\;{\sum\limits_{j = 0}^{M - 1}\;{\sum\limits_{i = 0}^{L - 1}\;{\left( {{I\left( {i,j,k} \right)} - \overset{\_}{I}} \right)^{2} \times {\sum\limits_{k = 0}^{N - 1}\;{\sum\limits_{j = 0}^{M - 1}\;{\sum\limits_{i = 0}^{L - 1}\;\left( {{T\left( {i,j,k} \right)} - \overset{\_}{T}} \right)^{2}}}}}}}}}} & (8)\end{matrix}$

Here, L, M, and N are the number of voxels in the depth, width, andheight directions of the calculated absorption coefficient distribution,I(i, j, k) is the distribution of the target region in the 756 nmabsorption coefficient distribution, Ī is the average value ofdistribution in the 756 nm target region, T(i, j, k) is the distributionof the target region in the 797 nm absorption coefficient distribution,and T is the average value of distribution in the 797 nm target region.This enables calculation of normalized cross-correlation coefficientsfor I and T. While FFD has been used here as the deforming technique,other interpolation methods may be used, such as radius basis function(RBF) and so forth.

While the normalized cross-correlation coefficient has been used as thecost function, sum of squared distance (SSD), mutual information amount,or the like may be used, as long as a statistical amount representingimage similarity, and the statistical amount may be converted so thatthe images match each other more the smaller the value of the costfunction is at this time. The cost function may be calculated for theentire absorption coefficient distribution region, the user may extracta particular region, or a region may be set unique to the apparatus. Theentire region of the calculated absorption coefficient distribution isset as the target region here.

The dispersion value of the oxygen saturation was used as thestandardization term f(I, T). In this calculation method, oxygensaturation is calculated from the 756 nm absorption coefficientdistribution I and 797 nm absorption coefficient distribution Taccording to Expression (5), and the dispersion of oxygen saturationdistribution is calculated. However, there are body tissue portionswhich do not contain blood, so the oxygen saturation distributioncalculated here may be poor in accuracy if the dispersion value ofoxygen saturation is calculated for the entire region of the absorptioncoefficient distribution. This problem can be handled by calculating thedispersion value of the oxygen saturation corresponding to the bloodvessel portions, by binarizing portions corresponding to blood vesselswhen calculating, or weighting the oxygen saturation distribution withthe intensity of the absorption coefficient. Here, the oxygen saturationdistribution is weighted by the intensity of the absorption coefficient.Also, while Expression (7) has been used as the cost function E, but anyformat may be used as long as it is an expression with a standardizationterm such that the variance in oxygen saturation is reduced. At thistime, the oxygen saturation distribution is within the range of 0% to100%, so in a case where the calculated oxygen saturation at the time ofperforming deformation positioning such that the dispersion value issmallest is not within this range, this is a deformation positioningerror. Accordingly, a term may be added as a standardization term sothat the calculated oxygen saturation distribution is within this range.Also, a term may be added as a standardization term to suppress changein volume, so that the absorption coefficient distribution is notgreatly deformed.

The 756 nm absorption coefficient distribution is deformed by FFD, andin S206 the cost function is calculated between the deformed 756 nmabsorption coefficient distribution and the 797 nm absorptioncoefficient distribution. The 756 nm absorption coefficient distributionis repeatedly deformed to reduce the cost function, and in a case wherethe cost function is smaller than a threshold value determinedbeforehand, in S207 the deformation positioning ends since positioninghas been sufficiently performed. On the other hand, in a case where thecost function equal to or larger than the threshold value determinedbeforehand, deformation positioning is performed by repeating thepositioning processing. An arrangement may be made for thisdetermination where the number of times that the processing has beenrepeated is counted, and when the counter value reaches a present numberof times, the repeating processing ends at that point. This method isadvantageous in that the series of repeating calculations can beexpected to end within a certain amount of time, ensuring real-timenature of the entire system. In S208, the deciding unit 14 decides thepositional deviation between the probe and subject due to deformation ofthe subject and so forth, based on the plurality of statisticalinformation calculated in S203 and S206. The amount of deformation wherethe smallest dispersion is yielded from the multiple dispersions isdecided to be the positional deviation between the absorptioncoefficient distributions here (relative positional deviation betweenthe 756 nm absorption coefficient distribution and 797 nm absorptioncoefficient distribution). Processing from S209 and thereafter is thesame as in the first through third embodiments.

While absorption coefficient distributions of two wavelengths has beenused in the present embodiment, the present embodiment is applicable tocases of performing imaging using three or more wavelengths. The objectcan be achieved here by first performing deformation positioning on theabsorption coefficient distribution of two wavelengths using thetechnique as described above, and thereafter performing deformationpositioning between the absorption coefficient distribution regardingwhich deformation positioning has been completed and the absorptioncoefficient distribution of another wavelength.

Also, using an absorption coefficient distribution in the presentembodiment where the probe is scanned and the absorption coefficientdistributions of the pulses are added and averaged, whereby deformationpositioning can be performed among the averaged absorption coefficientdistributions.

Also, the plurality of distribution data may be subjected to both thecoordinate shifting described in the first through third embodiments,and the deformation processing described in the present embodiment. Thatis to say, at least one of coordinate shifting processing anddeformation processing can be executed.

Accordingly, the dispersion of the oxygen saturation is used as astandardization term, thereby correcting positional deviation among theabsorption coefficient distributions of multiple waveforms, so that thedispersion value of the oxygen saturation is reduced.

An advantage of the present embodiment is that, if relative positionaldeviation and deformation is occurring in the initial sound pressuredistribution and absorption coefficient distribution among manywaveforms, correction can be performed taking into consideration thenature of living tissue, without having to use an ultrasound apparatus.This enables not only parallel movement when correcting positionaldeviation and deformation, but also more complicated positionaldeviation and deformation to be corrected. Also, deformation which couldnot occur in living tissue is not performed during deformationpositioning, so good results are obtained regarding calculation accuracyof oxygen saturation.

Fifth Embodiment

Next, a fifth embodiment will be described. The photoacoustic apparatusaccording to the present embodiment is illustrated in FIG. 12. Ascanning unit 20 is a part which differs from the photoacousticapparatus according to the first through third embodiments. Other thanthat, the photoacoustic apparatus according to the present embodimentuses the same device configuration, so detailed description of theconfigurations will be omitted. Note however, there is difference as tothe first through fourth embodiments with regard to the processingperformed by the signal processing unit 40, so description will be madebelow focusing on the differences.

In the fifth embodiment, deviation in absorption coefficientdistribution occurring due to breathing and body movement at the time ofscanning with the probe is corrected at an absorption coefficientdistribution calculating unit. Thereafter, deformation positioning isperformed to correct the deviation of absorption coefficientdistribution wavelengths of pulses as to the generated absorptioncoefficient distribution, so that the dispersion value of oxygensaturation is the smallest, the method of which will be described. Thegenerated 756 nm absorption coefficient distribution is in a state whereabsorption coefficient distribution segments for each pulse arepositioned by deformation over all scanning positions. By performingdeformation positioning of 797 nm absorption coefficient distributionsegments as to the 756 nm absorption coefficient distribution, thedifference in absorption coefficient values among wavelengths is nolarger than if the 756 nm absorption coefficient distribution and 797 nmabsorption coefficient distribution are generated independent of eachother.

At the time of performing deformation positioning of the 756 nmabsorption coefficient distribution and 797 nm absorption coefficientdistribution segment, a constriction is set so that the dispersion valueof oxygen saturation is small.

The scanning unit 20 scans the light source 1 and probe 30. The lightsource 1 alone may be scanned, the probe 30 alone may be scanned, bothmay be scanned independently, and both may be scanned simultaneously.

In the steps in FIG. 13, first, in S301 the subject is irradiated withpulsed light of both wavelengths 756 nm and 797 nm, and the acousticwaves at a scanning position, which is an acoustic wave probe, areobtained and converted into digital signals. Acoustic waves generated bypulses irradiated at other scanning positions as well are obtained andeach converted into digital signals. Image reconstruction is performedfor each of the digital signals, and absorption coefficient distributionsegments regarding each of the pulses near the scanning position aregenerated. In the present embodiment, generating of the absorptioncoefficient distribution segments is performed at once in this step, butthe absorption coefficient distribution segments of one wavelength maybe performed in parallel during the deformation positioning process fromS303 to S309. Next, in S302, due to the scanning rate of the probe,there are places where the absorption coefficient distribution segmentsregarding the signals obtained by irradiation of the 756 nm wavelengthoverlap with each other, and from the arithmetic average of theabsorption coefficient distribution segments at the scanning positions,one large absorption coefficient distribution can be generated. At thetime of generating the one large absorption coefficient distribution,the absorption coefficient distribution segments may be positioned. Thesize of the absorption coefficient distribution segments correspondingto the pulses preferably is the range where the irradiation light of thepulses is cast, but may be a smaller region than the region where theirradiation light is being cast if deformation positioning of imagefeatures such as blood vessels to be positioned can be appropriatelyperformed, or may be the size of the entire absorption coefficientdistribution to be generated. In the present embodiment, the size of anabsorption coefficient distribution segment is a cubic region 40 mm ineach dimension, centered on the irradiation position of each pulse. In acase where the size of the entire absorption coefficient distribution tobe generated is to be used as the absorption coefficient distributionsegments, deformation positioning is preferably performed using onlyimage features near the pulse scanning position.

Any deformation positioning technique may be used for the deformingpositioning method, such as FFD or RBF at the overlapping portions ofthe absorption coefficient distribution segments. The evaluationfunction may be any, as long as a statistical amount representing imagesimilarity, such as normalized cross-correlation, mutual informationamount, SSD, or the like.

Deformation positioning of the overlapping portion of the absorptioncoefficient distribution segments is performed here so that thenormalized cross-correlation coefficient is largest, and an absorptioncoefficient distribution is generated for the overlapping portion byarithmetic averaging or the like. Thus, an absorption coefficientdistribution corresponding to one wavelength is generated.

FIG. 14 illustrates a periphery 50 of the absorption coefficientdistribution where absorption coefficient distribution segments of 756nm wavelength pulses have been integrated, a laser scanning locus 51,irradiation positions 52 of each pulse, and absorption coefficientdistribution segments 53 of the pulses. While drawn as a two-dimensionalimage, there is another dimension in the depth direction of the plane ofthe drawing, and three-dimensional absorption coefficient distributionsegments and absorption coefficient distributions are generated. Theabsorption coefficient distribution segments 53 of the pulses overalladjacent pulses, with deformation positioning being performed usingsimilarity such as correlation between the overlapping portions.

Next, in the steps of S303 through S309, the integrated 756 nmabsorption coefficient distribution that has been generated is subjectedto deformation positioning using the dispersion values of oxygensaturation of the absorption coefficient distribution segments at eachscanning position in 797 nm. Deformation positioning using thedispersion values of oxygen saturation is performed as follows. First,the integrated 756 nm absorption coefficient distribution and each ofthe pulse irradiation positions are saved in the first distributionobtaining unit 4. Next, at an absorption coefficient distributionsegment corresponding to a certain pulse in 797 nm and the pulseirradiation position thereof, a region corresponding to the absorptioncoefficient distribution segment corresponding to the certain pulse in797 nm is cropped out from inside the integrated absorption coefficientdistribution of 756 nm. Deformation positioning is performed among thecropped out regions.

At the time of deformation positioning, the dispersion value of theoxygen saturation is made to be smallest using the Expression (7), inthe same way as in the fourth embodiment. The dispersion value of theoxygen saturation corresponding to the blood vessel portions can becalculated by binarizing portions corresponding to blood vessels whencalculating, or weighting the oxygen saturation distribution with theintensity of the absorption coefficient. Here, the oxygen saturationdistribution is weighted by the intensity of the absorption coefficient.Also, Expression (7) has been used as the cost function E, but anyformat may be used as long as it is an expression with a standardizationterm such that the variance in oxygen saturation is reduced. Also, aterm may be added as a standardization term to suppress change involume, so that the absorption coefficient distribution is not greatlydeformed.

Now, in S307, the 797 nm absorption coefficient distribution issubjected to deformation processing by FFD, and a cost function iscalculated between the deformed 797 nm absorption coefficientdistribution segments and 756 nm absorption coefficient distribution. InS308, deformation processing of the 756 nm absorption coefficientdistribution is repeatedly performed so that the cost function will besmall, and if smaller than a threshold value determined beforehand,positioning is deemed to have been performed sufficiently, anddeformation positioning is ended. On the other hand, in a case where thecost function equal to or larger than the threshold value determinedbeforehand, deformation positioning is performed by repeating thepositioning processing. An arrangement may be made for thisdetermination where the number of times that the processing has beenrepeated is counted, and when the counter value reaches a present numberof times, the repeating processing ends at that point. This method isadvantageous in that the series of repeating calculations can beexpected to end within a certain amount of time, ensuring real-timenature of the entire system.

In a case where deformation positioning relating to the absorptioncoefficient distribution segments has ended in S308, the 797 nmabsorption coefficient distribution segment subjected to deformationpositioning is held as a part of the 797 nm absorption coefficientdistribution, and the steps from S303 to S308 are repeated at theabsorption coefficient distribution segment at the next scanningposition. In a case where deformation positioning of the 797 nmabsorption coefficient distribution segment as to the 756 nm absorptioncoefficient distribution at the next scanning position has ended, the797 nm absorption coefficient distribution generated one scanningposition earlier is subjected to arithmetic averaging, therebyintegrating the absorption coefficient distribution segments.

In a case where deformation positioning of the number of scans to begenerated has ended in S309, this state is one where the integrated 756nm absorption coefficient distribution and the integrated 797 nmabsorption coefficient distribution have been generated. Here, theamount of deformation yielding the smallest dispersion of the multipledispersions is decided to be the positional deviation among theabsorption coefficient distributions (relative positional deviationbetween the absorption coefficient distribution of wavelength λ₁ and theabsorption coefficient distribution of wavelength λ₂) (S310).Accordingly, comparison computation is performed between the obtainedabsorption coefficient distributions of the multiple wavelengths, and animage of the oxygen saturation distribution is generated in S311.Processing after S311 is the same as in the first through fourthembodiments.

While absorption coefficient distributions of two wavelengths has beenused in the present embodiment, the present embodiment is applicable tocases of performing imaging using three or more wavelengths, in the sameway as with the fourth embodiment. The object can be achieved here byfirst performing deformation positioning on the absorption coefficientdistribution of two wavelengths using the technique as described above,and thereafter performing deformation positioning between the absorptioncoefficient distribution regarding which deformation positioning hasbeen completed and the absorption coefficient distribution of anotherwavelength.

Performing this deformation positioning enabled deformation positioningto be performed of the 797 nm absorption coefficient distribution as tothe 756 nm absorption coefficient distribution of which the deformationhad been corrected. Thus, compared with an absorption coefficientdistribution obtained by adding the 797 nm wavelength absorptioncoefficient distribution segments without positioning, and an oxygensaturation distribution calculated among absorption coefficientdistributions obtained by adding the 756 nm wavelength absorptioncoefficient distribution segments without positioning, the deviationamong absorption coefficient distribution segments within the wavelengthis corrected, and moreover, positional deviation of correspondingabsorption coefficient distribution values among different wavelengthsis reduced, so the calculation accuracy of oxygen saturation improves.

An advantage of the present embodiment is that, if relative positionaldeviation and deformation is occurring in generated initial soundpressure distribution and absorption coefficient distribution among manywaveforms in a photoacoustic apparatus having a scanning unit,correction of relative positional deviation and deformation can beperformed taking into consideration the nature of living tissue, withouthaving to use an ultrasound apparatus. Accordingly, calculation accuracyof the absorption coefficient distribution of one frequency improves inthe generated initial sound pressure distribution and absorptioncoefficient distribution among many waveforms in a photoacousticapparatus having a scanning unit, and absorption coefficientdistribution segments of the other wavelength are matched to thisabsorption coefficient distribution, which enables not only parallelmovement when correcting positional deviation and deformation, but alsomore complicated positional deviation and deformation to be corrected,and moreover, correction can be performed taking into consideration thenature of living tissue for each pulse. Also, deformation which couldnot occur in living tissue is not performed during deformationpositioning, so good results are obtained regarding calculation accuracyof oxygen saturation.

Other Embodiments

Additional embodiment(s) can also be realized by a computer of a systemor apparatus that reads out and executes computer executableinstructions (e.g., one or more programs) recorded on a storage medium(which may also be referred to more fully as a ‘non-transitorycomputer-readable storage medium’) to perform the functions of one ormore of the above-described embodiment(s) and/or that includes one ormore circuits (e.g., application specific integrated circuit (ASIC)) forperforming the functions of one or more of the above-describedembodiment(s), and by a method performed by the computer of the systemor apparatus by, for example, reading out and executing the computerexecutable instructions from the storage medium to perform the functionsof one or more of the above-described embodiment(s) and/or controllingthe one or more circuits to perform the functions of one or more of theabove-described embodiment(s). The computer may comprise one or moreprocessors (e.g., central processing unit (CPU), micro processing unit(MPU)) and may include a network of separate computers or separateprocessors to read out and execute the computer executable instructions.The computer executable instructions may be provided to the computer,for example, from a network or the storage medium. The storage mediummay include, for example, one or more of a hard disk, a random-accessmemory (RAM), a read only memory (ROM), a storage of distributedcomputing systems, an optical disk (such as a compact disc (CD), digitalversatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, amemory card, and the like.

While the present disclosure has been described with reference toexemplary embodiments, it is to be understood that these exemplaryembodiments are not seen to be limiting. The scope of the followingclaims is to be accorded the broadest interpretation so as to encompassall such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No.2014-122535, filed Jun. 13, 2014, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. A photoacoustic apparatus comprising: a lightsource configured to generate a plurality of lights, each of theplurality of lights having a different wavelength from each other; aconverting element configured to receive photoacoustic waves generatedin a subject by irradiating the subject with the plurality of lights; afirst distribution obtaining unit configured to calculate a plurality ofpieces of property distribution information, corresponding to theplurality of lights, which each represents an optical absorptioncoefficient distribution, an initial sound pressure distribution, or anoptical energy absorption density distribution within the subject, theproperty distribution information being based on photoacoustic wavesgenerated by irradiating the subject with each of the plurality oflights; a second distribution obtaining unit configured to calculate, aplurality of times, a plurality of pieces of at least part of oxygensaturation distribution information within the subject by performing atleast one of coordinate shifting processing and deformation processingbetween the plurality of pieces of property distribution informationcorresponding to the plurality of lights; a spatial variance informationobtaining unit configured to calculate a plurality of pieces of spatialvariance information indicating a spatial variance, corresponding to theplurality of light, in the plurality of pieces of at least part of theoxygen saturation distribution information; and a decision unitconfigured to calculate positional derivation information between thesubject and the converting element in a case where the spatial varianceindicated by the spatial variance information is smaller than apredetermined value or the smallest from among the plurality of piecesof spatial variance information, wherein the second distributionobtaining unit is configured to calculate the oxygen saturationdistribution information using the plurality of pieces of the propertydistribution information after the at least one of the coordinateshifting processing and the deformation processing in according to thepositional derivation information.
 2. The photoacoustic apparatusaccording to claim 1, wherein the light source is configured to generatethe plurality of lights including at least light of a first wavelengthand light of a second wavelength that differ from each other, whereinthe converting element is configured to receive first photoacousticwaves generated in the subject by irradiating the subject with light ofthe first wavelength and second photoacoustic waves generated in thesubject by irradiating the subject with light of the second wavelength,wherein the first distribution obtaining unit is configured to: obtainfirst property distribution information which represents an opticalabsorption coefficient distribution, an initial sound pressuredistribution, or an optical energy absorption density distributioncorresponding to the first wavelength using first time-sequencereception signals output from the converting element by receiving thefirst photoacoustic waves, and obtain second property distributioninformation which represents an optical absorption coefficientdistribution, an initial sound pressure distribution, or an opticalenergy absorption density distribution corresponding to the secondwavelength using second time-sequence reception signals output from theconverting element by receiving the second photoacoustic waves, whereinthe second distribution obtaining unit is configured to obtain theoxygen saturation distribution information within the subject using thefirst and second property distribution information.
 3. The photoacousticapparatus according to claim 1, further comprising: a region settingunit configured to set a target region in the subject, wherein thespatial variance information obtaining unit calculates the spatialvariance information of the oxygen saturation distribution informationin the target region.
 4. The photoacoustic apparatus according to claim3, wherein the region setting unit sets the target region based on userinput.
 5. The photoacoustic apparatus according to claim 3, wherein theregion setting unit sets the target region based on values of theproperty distribution information.
 6. The photoacoustic apparatusaccording to claim 3, wherein the region setting unit sets a pluralityof pieces of the target regions at different positions in the subject.7. The photoacoustic apparatus according to claim 1, further comprising:a display control unit configured to cause a display unit to display animage of the oxygen saturation distribution information.
 8. Thephotoacoustic apparatus according to claim 1, wherein the spatialvariance information obtaining unit obtains a dispersion value as thespatial variance information.
 9. A method for processing a signal in aphotoacoustic apparatus that includes at least one processor, a lightsource and a converting element, the method comprising: causing thelight source to generate a plurality of lights, each of the plurality oflights having a different wavelength from each other; receiving, by theconverting element, photoacoustic waves generated in a subject byirradiating the subject with the plurality of lights; calculating, bythe at least one processor, a plurality of pieces of propertydistribution information, corresponding to the plurality of lights,which each represents an optical absorption coefficient distribution, aninitial sound pressure distribution, or optical energy absorptiondensity distribution within the subject, the property distributioninformation being based on photoacoustic waves generated by irradiatingthe subject with each of the plurality of lights having a differentwavelength from each other; calculating, by the at least one processor,a plurality of times, a plurality of pieces of at least part of oxygensaturation distribution information within the subject by performing atleast one of coordinate shifting processing and deformation processingbetween the plurality of the property distributions on opticalabsorption corresponding to the plurality of lights; calculating, by theat least one processor, a plurality of pieces of spatial varianceinformation indicating a spatial variance, corresponding to theplurality of light, in the plurality of pieces of at least a part of theoxygen saturation distribution; calculating, by the at least oneprocessor, positional deviation information between the subject and theconverting element configured to receive the photoacoustic waves in acase where the spatial variance indicated by the spatial varianceinformation is smaller than a predetermined value or the smallest fromamong the plurality of pieces of spatial variance information, andcalculating, by the at least one processor, the oxygen saturationdistribution information using the plurality of pieces of the propertydistribution information after the at least one of the coordinateshifting processing and the deformation processing in according to thepositional derivation information.
 10. A non-transitory computerreadable storage medium that stores computer executable instructions tocause a computer having at least one processor to execute a method forprocessing a signal in a photoacoustic apparatus that includes a lightsource and a converting element, the method comprising: causing thelight source to generate a plurality of lights, each of the plurality oflights having a different wavelength from each other; receiving, by theconverting element, photoacoustic waves generated in a subject byirradiating the subject with the plurality of lights; calculating, bythe at least one processor, a plurality of pieces of propertydistribution information, corresponding to the plurality of lights,which each represents an optical absorption coefficient distribution, aninitial sound pressure distribution, or optical energy absorptiondensity distribution within the subject, the property distributioninformation being based on photoacoustic waves generated by irradiatingthe subject with each of the plurality of lights having a differentwavelength from each other; calculating, by the at least one processor,a plurality of times, a plurality of pieces of at least part of oxygensaturation distribution information within the subject by performing atleast one of coordinate shifting processing and deformation processingbetween the plurality of the property distributions on opticalabsorption corresponding to the plurality of lights; calculating, by theat least one processor, a plurality of pieces of spatial varianceinformation indicating a spatial variance, corresponding to theplurality of light, in the plurality of pieces of at least a part of theoxygen saturation distribution; calculating, by the at least oneprocessor, positional deviation information between the subject and theconverting element configured to receive the photoacoustic waves in acase where the spatial variance indicated by the spatial varianceinformation is smaller than a predetermined value or the smallest fromamong the plurality of pieces of spatial variance information; andcalculating, by the at least one processor, the oxygen saturationdistribution information using the plurality of pieces of the propertydistribution information after the at least one of the coordinateshifting processing and the deformation processing in according to thepositional derivation information.